Towing vehicles are often required to tow objects which exert a strong resistance to towing. In the snowmobile industry, for example, tracked vehicles must pull massive trail groomers, heavily laden with snow, over winding, undulating trails of snow and ice. On ski slopes, tracked vehicles must tow snow groomers through snow and up steep slopes. Farm tractors must tow ploughs through earth and mud. Tanks and other military vehicles must tow heavy equipment over rough terrain.
In the above situations, and in many more, the towed objects exert significant resistance to towing. Where conventional towing hitches are used, such resistance adversely affects the towing vehicle's ability to change direction. That is to say, the force exerted by the towed object on the towing vehicle resists the towing vehicle's attempts to manoeuvre, with the result that the towing vehicle, if it can turn at all, is limited by a large minimum turning radius. Such a lack of manoeuvrability is generally undesirable and often unacceptable.
Another problem is that many tracked vehicles, as well as some wheeled vehicles, are steered by applying a brake to the track, tracks or wheels on one side of the vehicle. As a result, such vehicles often lose traction when attempting to turn while pulling heavy loads. In addition, such steering systems, often referred to as skid steering systems, are associated with significant wear on the vehicle's braking system.
It is known to enhance the turning ability of a towing vehicle by having a towed vehicle exert a turning moment on a towing vehicle which, in some circumstances, may be sufficient to steer the towing vehicle. In the snowmobile industry, for example, such a moment is known to be applied by attaching one or more hydraulic rams between a tractor and a trail groomer. In such an arrangement, the range of lateral movement of the trail groomer relative to the towing vehicle tends to be limited.
Another example of a towed object exerting a moment on a towing vehicle is found in Canadian Patent No. 401,229 (Davidson), which teaches a mechanism for steering a tractor with a single, centrally oriented track. The towed apparatus acts, in effect, as a rudder for the tractor.